A current transformer for self-powering of a circuit breaker should generally output a sufficiently high energy at a small primary current, and at the same time should also be able to limit the size of the secondary current when the primary current is high, so as to prevent the breakdown of components in a rectifying circuit due to a large outputted secondary current. FIG. 1 shows a current transformer in one embodiment based on the prior art. This is also provided with a bypass core 12 beside a main core 11, in order to limit the secondary current. Such a design can achieve the effect of limiting current, but in the case where the primary current carries a high-frequency signal or harmonic signal, relatively serious heating will occur in the main core 11 and bypass core 12 as a result. Such heating might have an effect on a measurement current transformer close to the energy current transformer, and the power loss will cause an increase in the overall energy consumption of the circuit breaker.